This invention relates to dynamoelectric machines with a superconducting rotor winding and particularly to the support, insulation, and cooling of the end turn region of the rotor winding.
Ying et al. U.S. Pat. No. 4,368,399, Jan. 11, 1983, which is herein incorporated by reference, discloses a superconducting rotor end turn support arrangement upon which the present invention improves.
The end turn windings of a superconducting generator rotor require methods of support, cooling, insulation and assembly much different than those of a conventional, i.e., nonsuperconducting rotor. One of the major reasons is the magnitude and direction of the forces on the winding. In addition to the normal radial load on a coil due to centrifugal effects, much larger magnetic forces are produced in the winding due to the increased number of ampere turns in the field winding of a superconducting rotor. These magnetic loads, which can be on the order of about 10,000 pounds per inch, have two components. One component is radially outward from the centerline of the rotor and the other component is normal to the radial component. The normal component is everywhere directed away from the pole, that is, axially outward from the U-shaped end turn. The radial electromagnetic loading adds to the radial loading due to rotation. These additional electromagnetic forces, not significant in a conventionally cooled rotor, require high strength forging material to adequately constrain the superconducting winding.
The superconducting winding is mechanically constrained in the rotor body in slots formed by rotor teeth and closed by wedges, similar to that portion of a conventionally cooled rotor. The additional requirements for cooling a superconducting winding in a rotor can be provided for by means such as that disclosed in Eckels U.S. Pat. No. 4,282,450, Aug. 4, 1981. Constraining the superconducting end winding requires still further improvement over a conventionally cooled rotor. The conventional method of brazing and blocking the end turns is not adequate to support the superconducting winding against the additional electromagnetic loads. There have thus been provided, as disclosed in the above-mentioned Ying et al. patent, extensions of the teeth, called end region support blocks, which support the winding in the end turn region. These end region support blocks are made of the same high strength rotor forging material as the rotor body in which the slots are located. Even with these high strength support blocks, however, small displacements between adjacent conductors due to elastic deformation of the coil will occur. These small displacements, which may be only of about 0.001 to 0.01 inch (about 0.025 to 0.25 mm.) between adjacent conductors can generate heat due to the rubbing that occurs. If this heat goes into the winding it can cause the superconducting winding to rise above its transition temperature and to be quenched, i.e., the winding may lose its superconducting properties.
As a consequence of the foregoing considerations, the cooling scheme and the structural support scheme for the end winding are essentially related. Coolant must be provided not only to cool the winding below its transition temperature when stable but also to keep it at this temperature by removing any heat generated by any motion of the superconductors in operation. The aim is therefore to provide a mechanically secure end winding assembly but still one in which the coolant, normally liquid helium, is present over substantial major portions of the superconductor surfaces with provision for the helium to flow freely through cooling channels in a circulation path. As superconducting rotors are intended to operate, a reserve of liquid helium is stored in the bore of the inner rotor. The helium is to be moved by rotational forces from the bore through holes in the pole head and out into the winding cooling channels providing a cold helium reserve for cooling within the confines of the end region support block. A natural convection system operating in a "g" field insures proper cooling of the winding. When any local heat generation occurs, the warmer helium in the hot spot area is recirculated to the cold storage volume at the sides of the coil. Due to the support and constraint requirements of the superconducting field winding, it is not possible to have the end turns totally floating in liquid helium with direct contact of the coolant with the entirety of the superconducting surface.
The structure as disclosed in the Ying et al patent helps to hold the end winding in place within the end region support blocks by a set of insulating spacers, one on each side of a coil. The insulating spacers as there disclosed would, however, limit coolant access to the exterior of the coil to an undesirable extent.
This invention addresses these considerations and provides an improved structure with intermittent supports that constrains the end turns to acceptable levels of motion under operating conditions and at the same time provides channels for the flow of liquid helium with substantial direct contact of coolant to adequately cool the superconductor.
The present invention employs end region support blocks of L-shaped cross section such as are disclosed in the above-mentioned Ying et al. patent and in addition utilizes sets of spacers that are both mechanically strong and electrically insulating on the inward and outward and under surfaces of the coil in a manner that permits coolant circulation in direct contact with the winding. It also provides for supporting the end turn in a position tilted toward the pole while utilizing only readily machined elements. The spacers comprise a rear set that extends radially behind the coil and a front set that extends radially in front of the coil. (By the rear or inward side of a coil is meant the side facing the pole and the front or outward side of a coil means the side facing away from the pole.) At the bottom of the coil are a set of bottom spacers that may be integrally formed with, for example, the rear spacers to provide L-shaped elements to which the front side spacers are keyed. In a preferred form, the front and rear spacers are tapered. The rear spacers are tapered from a thicker dimension at the bottom of the coil to a thinner dimension at the top of the coil. Conversely, the front spacers are tapered from a thicker dimension at the top of the coil to a thinner dimension at the bottom of the coil. Thus the front and rear spacers confine the conductive stack so that it is tilted inward toward the pole, such as by about 5.degree. to 10.degree., for more sufficient mechanical constraint and greater stability. There is a corresponding angle to the top surfaces of the bottom spacer and to the bottom surface of the top spacer.
Additional elements of the invention, provided in preferred forms, are sheets of side insulation to the front and rear of the coil between the front and rear spacers and the adjacent radially extending leg of the end turn support block to the front and rear of the particular coil. The side insulation is particularly beneficial, even though the spacers insulate the stack, because at the end of the spacers the taper to the smaller dimension is intentionally a very limited distance, such as about 0.1 inch (about 2.5 mm) that in some cases is not enough of a stand off distance to prevent current from arcing to the end region support block from the coil.
Other significant aspects of the invention concern the insulating support on the top side of each end turn. In accordance with this invention, a top insulating spacer is provided over each of the conductive stacks with means such as banding for compressively holding the top spacer against the coil and the coil against the bottom spacers and the radially inward, axially extending leg of the support block.
The top spacer has coolant flow passages in its inward and outward surfaces and its radially inward surface and is disposed with its top surface substantially flush with the top surface of the end region support blocks that are adjacent it on each side so that an end turn spacer cylinder that is attached to the support blocks fits closely around all the elements.